G protein-coupled receptors (GPCRs) are a group of plasma membrane receptors that transduce signals from extracellular ligands to signals in intracellular relay proteins, the heterotrimeric GTP binding proteins (G proteins). GPCRs represent one of largest and most diverse protein families in mammalian genomes. (Hutchings et al., 2010 mAbs 2:6, 594-606; Strader et al., 1994 Annual Review of Biochemistry 63:101-132). GPCRs are characterized by having an extracellular N-terminus, 7-transmembrane spanning (TM) domains and an intracellular C-terminus (Hutchings et al., 2010 mAbs 2:6, 594-606). In vertebrates, these receptors can be classified into families based on their sequence similarity within the 7 TM domains, Class 1 (rhodopsin-like family), Class 2 (secretin and adhesion family), Class 3 (the glutamate family), and Class F (frizzled family) (Hutchings et al., 2010 mAbs 2:6, 594-606; Fredriksson et al., 2003 Mol Pharmacol. 63(6):1256-72).
The parathyroid hormone receptor (PTHR) is a class 2 GPCR that activates the adenylyl cyclase/cAMP signaling pathway. Similar to all class 2 GPCRs that bind peptide hormones, the PTHR has a relatively large (˜160 amino acid)N-terminal extracellular domain, herein termed the N domain, that plays a major role in hormone binding (Shimizu et al., 2004 J Biol Chem. 280(3):1797-807). Two types of PTHR, type 1 (PTH1R) and type 2 (PTH2R) have been identified. PTH1R mediates the actions of two polypeptide ligands; parathyroid hormone (PTH), an endocrine hormone that regulates the levels of calcium and inorganic phosphate in the blood by acting on bone and kidney, and PTH-related protein (PTHrP), a paracrine-factor that regulates cell differentiation and proliferation programs in developing bone and other tissues (Gardella et al., 2015 Pharmacol Rev. 67(2): 310-337). PTH a principal regulator of bone remodeling and calcium ion homeostasis, exerts its effects by binding and activating the PTHR (Shimizu et al., 2004 J Biol Chem. 280(3):1797-807).